Don;t know if it was popo but I hate when people make unsignaled lane changes when you are about two feet off their bumper and they decide to do that gentle slide over in front of you. Worse, the switch again, and decide at the light to do the same thing. I hate it when people decide to halve or less the stopping distance as you are braking. You go from gentle stop to full on slam on the brakes because they decide they need that piece of the road - again, that no signal, slow slide in front of you at the last minute.
So yes, literally almost ran into them..
No indication it was an umarked, typically you can see the lights in the grill or rear window when you are close. This was just some asshat with the all black look,.